BF4, PF6, AsF6 and SbF6 salts of triarylsulfonium are known and commonly used up to now as polymerization initiators of high cationic photopolymerization initiation performance.
However, with methods proposed up to now for manufacturing a sulfonium salt, such as a method in which a sulfide and a sulfoxide are condensed in the presence of a strong organic acid such as methanesulfonic acid or an inorganic acid such as sulfuric acid, and then subjected to double decomposition in an aqueous solution of a BF4, PF6, AsF6 or SbF6 salt of an alkali metal or the like (see Japanese Laid-Open Patent Application S61-100557 and Japanese Laid-Open Patent Application S61-212554, for example), a bis-sulfonium salt having two sulfonio groups per molecule is produced in addition to a monosulfonium salt having only one sulfonio group per molecule.
Bis-sulfonium salts generally have higher photopolymerization initiation performance than monosulfonium salts, but they have low solubility in cationic polymerizable monomers or in dilution solvents that are used as needed, so when a sulfonium salt is added to and dissolved in these in the required concentration, there is the problem that a bis-sulfonium salt may precipitate and settle from the sulfonium salt solution over time.
Also, a cationic polymerizable compound containing a bis-sulfonium salt tends to become more viscous over time, which is a problem in that such compounds cannot be stored for an extended periods.
These problems can be solved by refining the bis-sulfonium salts away from sulfonium salts comprising a mixture of monosulfonium salts and bis-sulfonium salts by some means such as recrystallization from an organic solvent, but the problem with performing such refining is that it greatly diminishes the yield of the desired monosulfonium salt.
In view of this, in an effort to solve the problem of the production of a bis-sulfonium salt encountered with conventional methods for manufacturing a sulfonium salt, the inventors have already proposed a manufacturing method with which a monosulfonium salt is obtained as the main component, that is, a manufacturing method with which the desired sulfonium salt can be obtained directly, with condensing a sulfide and a sulfoxide in the presence of a strong acid such as HBF4, HPF4, HAsF4, or HSbF4 without conducting double decomposition (see Japanese Laid-Open Patent Application 2002-241363).
The prior art published information correlated to the invention of this application are the following.                Japanese Laid-Open Patent Application S61-100557        Japanese Laid-Open Patent Application S61-212554        Japanese Laid-Open Patent Application 2002-241363        